The invention relates to a catalytic alkylation process and apparatus. In one embodiment, the invention relates to the control of temperature in the hydrocarbon phase of an acid settler vessel in a catalytic alkylation process and apparatus.
Numerous processes are known in the prior art for alkylating an alkylatable hydrocarbon such as an isoparaffin with an alkylating agent such as an olefin in the presence of a catalyst. A successful commercial system involves the circulation of an acid-type catalyst, such as hydrofluoric acid or sulfuric acid, through a reaction zone, a separation zone, a cooling zone and back to the reaction zone.
In one such process for the alkylation of hydrocarbons, hydrocarbons are introduced into the acid catalyst in the lower portion of an alkylation reaction zone maintained at suitable alkylation reaction conditions. A stream comprising catalyst, alkylate product and unconsumed reactants rises through the reaction zone and passes from the upper portion of the reaction zone into a catalyst settling zone, wherein separation occurs between the hydrocarbon phase and the catalyst phase. Catalyst withdrawn from the catalyst phase of the settling zone is cooled and returned to the reaction zone. Such a process involves cyclic catalyst flow through the system. A hydrocarbon stream is withdrawn from the hydrocarbon phase for further processing, including fractionation of unreacted isoparaffin from the alkylate product.
It is known that the presence of excessive amounts of water in the acid catalyst is highly corrosive to the alkylation and catalyst handling systems. Although the acid catalyst is normally utilized in an essentially anhydrous condition, there is a tendency in alkylation systems for water to accumulate as the acid catalyst is repeatedly recycled through the system. It is also known that a material known as acid-soluble oil is produced in the alkylation reaction, and that this material acts as a diluent for the catalyst phase. In general, the production of acid-soluble oil is substantially in excess of that necessary or desirable for dilution of the catalyst.
Consequently, it is common for alkylation systems employing acid-type catalysts to include a so-called catalyst "rerun" system through which at least a portion of the catalyst is passed to remove water and acid-soluble oil. The rerun process can involve withdrawing a portion of the catalyst phase as it flows from the acid settler vessel back to the reactor. The withdrawn catalyst phase is heated to a temperature sufficient to cause separation into a liquid phase, comprising predominately water and some acid-soluble oils, and a vapor phase comprising predominately catalyst, unreacted feed materials and some alkylate. The heated catalyst phase is passed into a catalyst rerun separator column. Water and acid-soluble oil are withdrawn from the bottom of the rerun separator column while the vapor phase is recycled to the acid settler vessel.
Butadiene or other impurities present in the feed can result in higher rates of formation of acid-soluble oils in the alkylation reaction. The higher concentrations of acid-soluble oils in the alkylation reaction effluent require correspondingly higher throughputs in the rerun tower, which in turn results in more hot catalyst phase for return to the acid settler vessel. It is desirable that at least part of the hot catalyst phase be passed into the hydrocarbon phase of the acid settler vessel in order to utilize some heat from the hot catalyst phase (to maintain pressure). However, the heat of vaporization from condensation of the hot catalyst phase in the hydrocarbon phase can cause the hydrocarbon phase to undergo an excessive increase in temperature and pressure. Since the alkylation reactor is in a continuous loop with respect to catalyst flow, the introduction of high pressure acid into the reactor requires operation of the alkylate/isobutane fractionator at pressures high enough to allow the recycle isobutane to flow into the reactor. This is costly in terms of higher pressure equipment and higher pressure steam for reboiler duty. It would be possible to control the temperature of the hot catalyst phase by passing it through an external cooler, but such a cooler designed for hot acid cooling would be expensive to purchase and maintain.
It would therefore be highly desirable to provide a means for controlling the temperature of the hydrocarbon phase in the acid settler vessel without the use of expensive cooling equipment for the hot rerun catalyst. It would also be desirable to control the pressure of the hydrocarbon stream entering the alkylate/isobutane fractionator.
It is therefore an object of the invention to provide a catalytic alkylation process and apparatus. In one embodiment, it is an object of the invention to inexpensively prevent temperature and pressure buildups in the acid settler vessel of a catalytic alkylation system.
These and other objects and advantages of the invention will be apparent from the following detailed description of the invention.